Postmenopausal women with this type of breast cancer, known as ductal carcinoma in situ (DCIS), should receive drugs such as tamoxifen and anastrozole to keep the disease from returning, Mortimer said.

One study followed nearly 3,000 women who had hormone-receptor positive DCIS breast cancer and were given the two drugs for five years after surgery. It was led by the Wolfson Institute of Preventive Medicine at Queen Mary University of London, England.

Although women who took anastrozole had a slightly higher chance of breast cancer returning than those who took tamixofen, it was not considered significant. Women taking anastrozole experienced fewer ovarian cancers and nonmelanoma skin cancers. They also had fewer deep vein thromboses and gynecological issues than women who took tamoxifen. However, women who took anastrozole had more strokes, fractures and musculoskeletal issues than those who took tamoxifen.

Another study on DCIS breast cancer by UCLA found that patients who used both drugs reported no differences in energy levels, depression and other quality-of-life measures. Those who took tamoxifen reported more severe hot flashes. Those on anastrozole had more severe vaginal dryness and muscle and joint pains.

Commenting on both studies, Mortimer told HealthDay that the results of the new DCIS research "are exactly what we see in invasive breast cancer.”

DCIS has received attention recently "because it is not an invasive cancer, yet is treated as aggressively," she added.

Some medical experts recommend holding off on DCIS treatment until there are signs of invasive cancers, but Mortimer says the recent research reveals otherwise.

"The fact that DCIS and invasive cancer both respond to endocrine therapy [tamoxifen and anastrozole] suggests that we should treat DCIS, so that invasive cancers don't develop," she said.

"Both of these hormonal therapies are effective and if one causes side effects, switching to the other is reasonable," she added.